Put HttpProxyClientSocket into a pool.
This CL requires http://codereview.chromium.org/2799036
- Cleanup the HttpProxyClientSocket interface a touch.
- Make HttpAuthController reference counted.
- Enable ClientSocketPool to return recoverable connections.
BUG=42795
TEST=existing unit tests
Review URL: http://codereview.chromium.org/2817033
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52104 0039d316-1c4b-4281-b951-d872f2087c98